Environmental interests, governmental regulations and incentives, and an increasing worldwide demand for energy are resulting in increased interest in renewable energy sources, such as hydrocarbon fuels derived from biological raw materials. In the manufacture of diesel fuels, one area of interest has been production of suitable diesel fuel via processing of vegetable oils and animal fats that contain triglycerides of fatty acids. Triglycerides contain three linear and mostly saturated hydrocarbon chains (normally 8 to 22-carbon atoms) that are linked together by an ester backbone. When the ester backbone is removed, the remaining linear hydrocarbon chains correspond chemically to hydrocarbons typically present in mineral (i.e., conventional) diesel fuels.
One conventional approach for converting vegetable oils or other fatty acid derivatives into liquid fuels in the diesel boiling range is by a transesterification reaction with an alcohol in the presence of a catalyst, such as sodium hydroxide. The obtained product is a fatty acid alkyl ester, and typically is a fatty acid methyl ester (FAME). While fatty acid alkyl esters have many desirable qualities, such as high cetane, there are issues associated with their use directly as diesel fuels. Fatty acid alkyl esters typically have poor cold flow properties due to a large weight percentage of straight chain hydrocarbons. Additionally, fatty acid alkyl esters often have low oxidation stability related to the presence of ester moieties and unsaturated carbon-carbon bonds.
Hydrogenation of vegetable oils or other fatty acid derivatives by co-processing with mineral diesel feedstocks is another approach for conversion of biologically derived feeds to hydrocarbon liquids in the diesel boiling range. This method removes undesirable oxygen by hydrodeoxygenation or hydrodecarboxylation reactions, and saturates the unsaturated carbon-carbon bonds present in feed molecules.
Hydrodeoxygenation and/or hydrodecarboxylation reactions are in many ways similar to other forms of hydrotreating currently used in refining of mineral hydrocarbon feedstocks, and therefore can potentially be practiced using existing infrastructure. However, hydrodeoxygenation reactions are highly exothermic relative to hydrodesulfurization and also require relatively large amounts of hydrogen. The excess heat generated by the hydrodeoxygenation reaction combined with the high levels of required hydrogen can lead to undesirably high reaction temperatures or low hydrogen availability in the feed stream during hydroprocessing. These undesirable conditions can lead to increased formation of unwanted side reaction products and coking of catalyst. Unwanted side reactions, such as cracking, polymerization, ketonization, cylization and aromatization decrease the yield and the beneficial properties of a diesel fraction. Additionally, unsaturated feeds and free fatty acids in triglyceridic biologically derived oils may also promote the formation of high molecular weight compounds that are not desirable in a diesel fuel. Therefore, there is a need for an improved process for refinery hydrotreatment of hydrocarbon streams that include a biologically derived feedstock, such as vegetable oils and/or animal fats.
Still another conventional approach to producing a diesel fuel including a biologically derived feedstock is to separately process both a mineral hydrocarbon feedstock and the biologically derived feedstock. The processed feedstocks can then be blended to produce a desired diesel fuel. While separate processing allows preferred conditions to be selected for each feedstock individually, this strategy requires significant additional equipment footprint in a refinery, as dedicated process trains are required for both feedstocks. Thus, this solution is not favorable from a cost standpoint.
Separately, regulatory requirements continue to reduce the level of sulfur that is permitted in diesel fuels. In order to meet worldwide regulatory standards, processes are needed that allow for production of diesel fuel with 10 ppm or less of sulfur.
EP 1693432 describes a process for production of a diesel fuel that includes hydrotreatment of a feedstock containing from 1-75% of a vegetable oil, with the balance of the feedstock being a mineral hydrocarbon feed. The mixed feedstock of vegetable and mineral oil is co-processed in at least one hydrotreatment stage. Sulfur levels in the resulting diesel fuel are not disclosed in EP 1693432.
U.S. Pat. No. 4,992,605 describes a process for hydrotreating various types of oils of biological origin for use as diesel fuel extenders. The hydrotreated oils are then separated and diesel component is removed for mixing with a conventional diesel fuel.
EP 1741768 describes a process for producing diesel range hydrocarbons from bio oils and fats. The feedstock for processing includes a bio oil or fat and a diluting agent. The diluting agent can be a traditional hydrocarbon stream. The combined bio oil or fat and diluting agent feedstock is then passed into a reactor at two different location. The first bio oil or fat and diluting agent stream enters the reactor above the first reaction bed, while the second stream, also containing bio oil or fat and diluting agent, enters the reactor downstream from the first reaction bed.